One in six ischemic strokes is due to cardiogenic embolism. The current knowledge about the natural history, diagnosis, prevention, and treatment of cardioembolic stroke is reviewed. Nonrheumatic atrial fibrillation is the most frequent substrate for brain embolism; patients with this dysrhythmia have a fivefold increase in the risk of stroke. Ischemic heart disease (25%), rheumatic mitral stenosis (10%), and prosthetic cardiac valves (10%) are other major sources of cardioembolic stroke. Mitral annulus calcification and calcific aortic stenosis infrequently serve as cardioembolic sources. As cardiac sources of embolic stroke are often identifiable prior to stroke, many of these strokes are potentially preventable.